Kauralexins are a class of ent-kaurene-related diterpenoid defense compounds found in maize. They are produced in response to a variety of biotic threats, including the larvae of the European corn borer Ostrinia nubilalis and fungi such as Rhizopus microsporus and Fusarium graminearum. Jasmonic acid and ethylene can also work synergistically to promote kauralexin production. Kauralexins can also be detected in the scutella of 19 diverse maize inbred lines suggesting that these compounds are broadly or universally expressed among all maize varieties [Schmelz11].

Importantly, at low concentrations, kauralexin A3 and kauralexin B3 reduce the growth of R. microsporus and C.graminicola. In addition, in choice and nonchoice assays Ostrinia nubilalis consume less tissue treated with these compounds than untreated control tissue, indicating that kauralexins also have anti-feedant properties [Schmelz11].

About This Pathway

Although many of the compounds in this pathway have been identified experimentally, notably kauralexin A1, A2, A3, and B1, B2, and B3 [Schmelz11] the overall pathway scheme is proposed largely based on the sequence of enzymatic activities observed in other terpenoid biosynthetic routes, such as juvenile hormone III biosynthesis I.

Several lines of evidence suggest that kauralexin production depends on the activity of An2, including the increased level of An2 transcripts that accumulate prior to kauralexin accumulation [Schmelz11].

Tranformation of ent-CPP to ent-kaurene and ent-isokaurene likely requires the activity two different synthases. In rice, these labdane-related diterpene cyclases show show quite stringent specificity. For example, OsKS1 (kaurane synthase 1) produces ent-kaurene whereas OsKSL5/6 (kaurane-synthase-like 5/6) appears to primarily catalyze the formation of ent-isokaurene [Xu07a]. Maize may have similarly stereospecific kaurene synthases.

The remainder of the reactions in the pathway are likely catalyzed by one or more cytochrome p450 enzymes and the predicted NADPH cofactors and oxygen inputs have been included on that basis. It is likely that these p450 enzymes would catalyze the formation of an alcohol and then its conversion to an aldehyde and finally a carboxylic acid to give rise to kauralexin A1 and kauralexin B1. The same series of reactions would likely be repeated on a different carbon atom to produce first two alcohols (not shown) followed by aldehydes kauralexin A3 and kauralexin B3 and finally the dicarboxylates kauralexin A2 and kauralexin B2. This series of reactions has been observed for several other p450 enzymes that act in plant secondary metabolism, such as those that participate in costunolide biosynthesis and gibberellin metabolism [Prosser02, Nguyen10b, Helliwell01a].

Identification of all of the unknown enzymes and the intermediate compounds that participate in kauralexin biosynthesis will lead to a better understanding of this overall pathway and its role in maize defense against fungi, herbivory, and other biotic threats.